PARASITISM OF PHOTOSYNTHETIC DINOFLAGELLATES IN A SHALLOW SUBESTUARY OF CHESAPEAKE BAY, USA

Rhode River (USA) populations of the red-tide dinoflagellates Gymnodin ium sanguineum Hirasaka, 1922, Gyrodinium uncatenum Hulburt, 1957, and Scrippsiella trochoidea (Stein) Loeblich III, 1976, were commonly inf ected by their parasitic relative Amoebophrya ceratii Cachon, 1964, du ring the summer of 1992. Mean infection levels were relatively low, wi th data for vertically integrated samples averaging 1.0, 1.9, and 6.5 % for G. sanguineum, G. uncatenum, and S. trochoidea, respectively. Ho wever, epidemic outbreaks of A, ceratii (20 to 80 % hosts parasitized) occurred in G. uncatenum and S. trochoidea on several occasions, with peak levels of parasitism associated with decreases in host abundance . Estimates for parasite induced mortality indicate that A. ceratii is capable of removing a significant fraction of dinoflagellate biomass, with epidemics in the upper estuary cropping up to 54 % of the domina nt bloom-forming species, G. uncatenum, daily. However, epidemics were usually geographically restricted and of short duration, with daily l osses for the 3 host species due to parasitism averaging 1 to 3 % over the summer. Thus, A. ceratii appears capable of exerting a controllin g influence on bloom-forming dinoflagellates of the Rhode River only w hen conditions are suitable for production of epidemic infections. Int erestingly, epidemics failed to occur in multiple dinoflagellate taxa simultaneously, even when alternate host species were present at high densities. This observation, along with laboratory experiments demonst rating that parasites isolated from G. sanguineum were unable to infec t G. uncatenum, S. trochoidea, and Ceratium furca, suggests that the d inoflagellate taxon A. ceratii may represent a cluster of relatively h ost-specific species.